1. Field of the Invention
The present invention relates to improvements in the process for vitrifying waste materials, and more particularly, to improvements especially for the vitrification of radioactive waste materials by changing the chemical form and physical characteristics of additives that are added to the waste material to obtain a suitable vitrified product, such that the reactions between the additives and the waste materials suppress the formation of undesirable secondary phases.
2. Description of Related Art
Large quantities of radioactive nuclear waste materials that are produced as byproducts from activities such as weapons programs, nuclear fuel recycling, medical isotope production, are stored in various countries around the world. Vitrification of these waste materials to produce a durable glass product is the preferred approach for treating and disposing of these waste materials because of the high durability of the glass waste form as compared to other waste forms such as grout. Vitrification of radioactive high level waste (HLW) to produce a borosilicate glass product is the internationally accepted waste treatment method. Vitrification of the waste materials is done in melters such as Joule Heated Ceramic Melters (JHCM), Induction Melters (IM), Cold Crucible Melters (CCM), and the like.
In waste vitrification, the waste materials are mixed with appropriate amounts of raw materials, known as “additives”, and melted at a high temperature (typically 1150° C. for JHCMs, typically somewhat lower for IMs, and typically somewhat higher for CCMs) in order to produce glass products that meet pre-specified product quality requirements. The product quality requirements mostly relate to the chemical durability of the glass product as measured by standard test procedures.
Many waste materials are stored in tanks in slurry or solution form. The additives have been used either in the form of pre-melted chemicals (frit) with a specified composition, or raw materials in the form of minerals or chemicals. The additive minerals or chemicals are commonly referred to as glass forming chemicals (GFCs). In either case, the waste materials with the additives are designed to melt and form a glass product with a predetermined oxide composition. The waste materials mixed with the appropriate amounts of frit or GFCs are referred to as melter feeds. These materials may be mixed either outside or inside the melter.
In many situations, the melter feed is introduced from the top of the melter to the melt pool surface so that a layer of feed material covers the hot glass melt underneath. This layer is commonly referred to as the cold-cap. The cold-cap extends from partially melted melter feed in contact with the melt pool to unreacted melter feed at the top. In other situations such as batch-style induction melters, the melt zone progresses from the hot wall on the outside into the bulk. Consequently, in this case also there is a boundary zone between predominantly melted material and predominantly unmelted feed material. This zone is also referred to as a cold cap. Reactions in the cold-cap, and controlling these reactions, are the key approach to mitigating undesirable secondary phase formation.
Due to the complexity of the waste materials, and the number of different constituents, persistent secondary phases are often formed during the melting process. The extent of formation of these secondary phases depend on various factors such as concentrations of troublesome constituents, the types of additives, waste processing rates, processing temperatures, etc. One of the common waste constituents that show a tendency to form secondary phases during nuclear waste vitrification is molybdenum. Formation of secondary molybdate phases is undesirable because they lead to both processing and product quality issues. Molybdate phases are easily leachable, leading to unacceptable product quality. In addition, these secondary phases tend to selectively incorporate high concentrations of other components such as cesium leading to additional product quality issues. Alkali molybdate phases can accumulate on the melt surface causing excessive corrosion of melter components that are in contact with this phase. Alkaline earth molybdates tend to sink to the bottom of the melter and accumulate. The accumulation of molybdate secondary phases at the melter bottom can cause problems with glass discharge, especially for melters that use a bottom discharge.
The molybdate phase formation is a result of the high molybdenum concentration in the HLW feed to the vitrification facility in combination with complex reaction kinetics in the cold-cap. The molybdenum secondary phase formation initially occurs near the interface between the molten glass pool and the cold-cap. Our studies showed that kinetically controlled cold-cap conversion processes are responsible for the generation of molten molybdate salt, rather than solubility limits of molybdates in the underlying glass melt.
When the melter feed is prepared by mixing glass frit with the HLW material in slurry form, or when HLW is fed continuously and glass frit is fed periodically in small batches to the center of the cold-cap, the molybdate secondary phase tends to form before the underlying glass melt reaches saturation with respect to molybdate. If the feed chemistry can be altered to suppress the molybdate secondary phase formation until its concentration reaches close to the solubility limit, the amount of waste incorporated into unit amount of glass produced (waste loading) can be increased leading to substantial cost savings in HLW treatment and disposal.
Sulfur is another component in waste streams that at high concentrations causes the formation of secondary sulfate layers during the melting process. The sulfate concentration in the glass that causes secondary phase formation can be as low as 0.5 wt % in certain glass compositions. In this case also, the secondary sulfate layer is formed well before the underlying glass melt reaches sulfate solubility limit. Again, improvements to the feed chemistry by judicious choice of additives to delay the formation of sulfate secondary phases until the sulfate concentration reaches close to the solubility limit, has enormous economical advantage. The same principles should be effective for mitigating other troublesome salt-forming species such as chlorine, fluorine, chromium (chromate), and phosphorous.
Accordingly, it is an object of the present invention to provide improved efficiency for vitrification of waste material, especially for radioactive waste materials.
It is a further object of the present invention to reduce the tendency to form secondary phases during vitrification by tailoring the glass former additives, thus increasing the amount of waste incorporated into unit amount of glass produced.